I use a cup or two of vinegar and a gallon of water. Hose down the deck with a garden hose and a spray nozzle to get the loose dirt washed out, then dip a scrubbing brush in the water and scrub the whole deck down. Rinse with the hose again, and open all the boxes. Once they are dry if you put a little fabric softener in a spray bottle with water and mist them a little it'll keep the knap a little softer. Then I use the scrub brush again going against the grain to get the grain brushed standing back up.
"If worms carried pistols, birds wouldn't eat 'em."
- Darrell Royal